Reading the board across from you
Before a fight, read the board you are about to meet: which side their carry hits first, where your best tank belongs, and which of their units leap.
Checked for patch 18.4. The numbers come from the live Set 18 data and follow every patch.
The idea
A board is not placed once. The board across from you changes from fight to fight, and the hexes that keep your carry alive against one opponent can hand it to the next. Before a fight that matters, look at the board you are about to meet, or at the strongest boards left in the lobby, and place against them.
Start with their carry. Their board meets yours turned around, so a unit on the left of their board stands across from the right of yours. It starts on the closest of your units on that side, and that unit takes its damage first. Make it your best tank, the one with the defensive items. Every front line has a stronger side and a weaker side, and the stronger one should face their damage.
Then aim your own damage. Your carry hits what stands on its side first; if their best tank waits there, your carry spends the opening seconds on the unit built to absorb them. Flipping your board, each unit to the other end of its row, is the quickest way to change who meets whom. When you cannot tell which side matters, keep your best tank in front of your own carry, so anything that reaches your carry has gone through it first.
Last, read their abilities, not only their units. Some units leap over the front line on their own, to an enemy they choose within a few hexes: the farthest, or the one with the least health. Their ability says how far and whom, and the hexes count from wherever the unit stands when it casts, usually inside your front line by then. Against them, keep a unit beside your carry, or move the carry so the unit they reach first can take it.
See it live
Your board against a current board with a unit that leaps. Their carry is selected: see whom it goes for first. Tap any unit to read it, and flip your board to compare.
Their carry, Kha'Zix, goes for your Krug, your Gnar and your Brambleback first. That is your best tank (marked Tank): its damage starts on the unit built to take it.
Their Kha'Zix, range 1, goes for your Krug, your Gnar and your Brambleback first, 3 hexes away. Nothing is in range before anyone moves: it walks first.
Its ability leaps to one enemy within 3 hexes. From where it starts, that is your Krug, your Gnar and your Brambleback; every hex it walks forward reaches a hex deeper.
Taste Their Fear: Leap to the farthest enemy within 3 Hexes, dealing 265/370/535 magic damage. If they have no adjacent allies, deal 285/410/605 magic damage instead and gain 10 mana.
Tap any unit to read it. Lit hexes: range and leap ; the units it goes for first are ringed.
The rule of thumb
Scout before the fight: put your best tank where their carry hits first, and keep your carry out of what their leaps reach.
Check yourself
Scouting, you see their Kha'Zix in the front row, on the left of their board. When you fight, which side of yours does it face?
Pick an answer.